In Legendary, he waited until the official release because he said he had too much on his plate at the time, but had access to the map before officially released. I am assuming he is a beta tester but it seems he has a lot on his plate right now with the village/lens/nether hub/seek the beast.

Well kinda, the thing with legendary was that Zisteau was making his own custom texture pack from scratch for the map. The texture pack was taking longer then he thought it would which was actually prolonging the release of the map to the public, as vechs wished to wait for zisteau to record his first episode so that they could release it simultaneously. Eventually Zisteau gave vechs the ok to release it without him because he wanted to get that texture pack just right, so he started a bit after release.

Not really how it works. How it worked previously when Z beta tested Vech's maps such as sea of flame, kaizo caverns or legendary, is that the beta testers get early access to the map and the first episode of the series is released at the same time as the map is.

Vechs asked him if he wanted to and he said yes, so I think so. plus because Zisteau is really the whole reason Super Hostile became famous (he was Vechs' first beta tester), I think he will always be welcome to
EDIT: or maybe not considering Zisteau may want to do a LP of the complete map with a new texturepack to record with. if so, would he want it spoiled?

I think you are confused by the term beta tester in this instance. Yes, it is early access to the map. However they aren't just walking through the map testing things, looking for flaws like a traditional beta tester. It is their legitimate playthrough. They just get to start their series before the map is released essentially. However they still fit the role of a Guinea pig.

Zisteau and vechs? Yeah. Zisteau was doing a lp of survival island, and vechs discovered his channel and started watching the series. This was before the whole super hostile series of course, back when neither one were particularly known. After watching zisteau for a while, vechs asked him if he would like to try out a map he was working on, which was kind of like survival island only much more difficult. This map was sea of flame. Zisteau wasn't sure at first because he already had a couple series going but decided to take a look, he saw the picture and knew he had to try it. Soon after that the first episode was released and at the same time the Super hostile thread.

during when Zisteau's channel was still new and he was doing his survival island LP, Vechs contacted him saying that he liked his videos and asked if he wanted to beta test a map that he made (sea of flame). Zisteau nearly passed it up, but Vechs sent him a screenshot of the beginning and Zisteau accepted. They then coordinated it so that the first episode of Zisteau's LP went up the same day as Vechs put the map up for download...So Vechs discovered Zisteau

Vechs indicated that he wasn't planning on sending beta tests out to LP'ers this time around, since most people see it as a full release, which it isn't. I am quite sure that Zisteau isn't already playing it.

EDIT: Don't get me wrong, I can't wait to see it, but after the trouble Vechs had with the BD II beta he said he wasn't planning on doing more of that. SH:IM is probably almost done and Zisteau will get a version that is done instead of a beta.

TL;DR: Zisteau will not get a beta but will also not have to wait for public release, I don't expect him to have it.

Edit to your edit: "A version that is done instead of a beta." If it's a version that he gets before everyone else to play with and make mistakes that can get fixed before the final release to the public... that is a beta.